In recent years, the trend of higher output power of semiconductor devices such as laser diode device (referred to as “LD” hereinafter) and light emitting diode device (referred to as “LED” hereinafter) is shown. With such trend, there occurs a problem that heat generation from the devices is increased to shorten the lifetime of the devices. To solve such a problem, aluminum nitride substrates having high heat dissipation property have come to be used as the substrates for device mounting.
In the use of the aluminum nitride substrate for device mounting, it is necessary to form an electrode or a wiring on the substrate surface in order to bond the device or supply electric power to the bonded device. Formation of the electrode or the wiring is carried out by forming a conductor layer that is designed so as to have a prescribed shape on the substrate surface using so-called metallization technique. As the metallization technique generally used in this case, a co-fire process or a post-fire process is known.
The co-fire process is a process comprising printing a metallizing paste containing a high-melting point metal in a shape of a prescribed pattern on a surface of an aluminum nitride green sheet, then firing the green sheet and the paste at the same time to bond the high-melting point metal to the substrate surface, and then if necessary, forming another metal layer thereon by plating or the like (see patent documents 1 and 2). The post-fire process is a process comprising printing a metallizing paste in a shape of a prescribed pattern on a sintered aluminum nitride substrate having been previously fired, then baking the paste to bond the high-melting point metal layer to the substrate surface, and then if necessary, forming another metal layer thereon by plating or the like (see patent document 3).
On the other hand, the substrate for mounting LD or LED generally has an extremely small size (e.g., 3 mm×3 mm), and for producing such a substrate, it is general from the viewpoint of production efficiency to adopt a process comprising forming a wiring pattern, in which a large number of “wiring patterns of individual substrates for device mounting” (also referred to as “wiring pattern units” hereinafter) are orderly arranged, on a surface of a multiple-piece-forming substrate of a large size and then cutting the substrate along boundaries of the wiring pattern units (said process being also referred to as a “multiple-piece-forming process” hereinafter) (see patent document 4). In this multiple-piece-forming process, many substrates can be produced at a time by regularly arranging the wiring pattern units in the form of a lattice and by cutting the substrate along the longitudinal and lateral boundaries that are both linear lines.
The co-fire process has an excellent feature that the high-melting point metal layer can be bonded to the substrate surface with a relatively high adhesion strength, but in the case where the multiple-piece-forming process is adopted, such non-uniform shrinkage of the substrate as shown in FIG. 1 takes place in the sintering step, and it is difficult to control dimensional accuracy. Hence, there occurs a problem that the yield cannot be increased. That is to say, in the multiple-piece-forming process, it is preferable from the viewpoint of production efficiency to densely arrange as many wiring pattern units as possible on a large substrate. In the co-fire process, however, the substrate is deformed into a shape of a star as shown in FIG. 1, so that when a large substrate is used, (i) a wiring pattern unit arranged in the vicinity of the circumference of the substrate suffers change in shape of the wiring pattern, and besides, (ii) even if the wiring pattern units are orderly arranged in the form of a lattice before sintering, rows of the wiring pattern units are curved after sintering. In the case where the substrate is cut linearly, therefore, some wiring pattern units pass over the boundary region, and the wiring pattern is damaged. In FIG. 1, the shape of a substrate 2 before firing (sintering) is indicated by a dotted line, and the shape of a substrate 1 after firing (sintering) is indicated by a solid line.
Although means to solve the above problem have been proposed, but control of dimensional accuracy to a satisfactory level has not been achieved yet. For example, there has been proposed as such a means in which to a part of an outer surface of a substrate, a pressure or a force of constraint of such a range that sintering shrinkage of the surface is not substantially brought about is applied in the firing step, and at the same time, a magnitude of creep occurring on the material is controlled to compensate firing shrinkage of a surface (free surface) of the sintering product to which a pressure and/or a force of constraint has not been applied, whereby the final outer shape of the sintering product is controlled (see patent document 5). However, even if such a special means is taken, realization of dimensional accuracy of about ±50 μm is the limitation in the region of about 50 mm square (50 mm×50 mm, referred to as “□50 mm” hereinafter), and in the region of about □100 mm, realization of dimensional accuracy of about ±100 μm is the limitation (“□” represents a square).
In the post-fire process, the metallizing paste is printed on the previously sintered substrate and then baked, so that shrinkage of the substrate does not take place in the baking step and the aforesaid problem of dimensional accuracy dose not occur. However, the bonding strength of the high-melting point metal layer formed on the sintered aluminum nitride surface in the post-fire process is not necessarily sufficient. For example, the aforesaid patent document 3 discloses technique of adding a specific component to the high-melting point metal paste to increase a bonding strength of the metallization layer, but the bonding strength is about 2 to 4 kg/mm2.
Patent document 1: Japanese Patent Laid-open Publication No. 2003-342089
Patent document 2: Japanese Patent Laid-open Publication No. 2003-179467
Patent document 3: Japanese Patent Laid-open Publication No. H8-34686
Patent document 4: Japanese Patent Laid-open Publication No. H8-239286
Patent document 5: Japanese Patent Laid-open Publication No. H5-283272.
As described above, there has not been known so far a multiple-piece-forming metallized substrate which is used for producing metallized aluminum nitride substrates preferably used as, for example, substrates for mounting LD or LED, is excellent in dimensional accuracy of wiring pattern units themselves and arrangement thereof and has a high bonding strength of a wiring pattern.
Therefore, it is an object of the present invention to provide a metallized aluminum nitride substrate satisfying such requirements as mentioned above.